How to spec a solar power system…

When mains power isn't available the sun provides us with a perfect source of power. But selecting a fit for purpose solar power system can be a complicated consideration.

What size battery is required? What size solar panel? What battery technology? Which charge controller?

The calculations are complex and the efficiency of the technology and devices used can have a massive impact on the systems performance, as can the installation location and skyline restrictions.

If we start with the known elements, we need to consider what voltage the device, or devices, we are powering require, and what their power consumption is. Data sheets detail these in a variety of ways and are not always clear as to what the power consumption will be. In order to calculate the suitability of the solar power system we need to know how many Ah's the device, or devices, will consume per day.

Depending on the unit of measure the data sheets provide, we can convert this to Ah using the power consumption calculator below.

Once we know the Ah/day we require, it would be fantastic if we could then just match this to a battery size to provide the desired number of days power should the weather restrict the solar charge performance. However, depending on the battery technology and voltage our devices require, we need to take in to account the efficiency loss from converting or inverting the power supply, the temperature impact on battery capacity and usable capacity of the battery.

If we start with the battery technology, we detailed in a previous post about the perfomance differences between LiFeP04 and Sealed Lead Acid/AGM batteries.

Sealed Lead Acid/AGM, although cheaper, are significantly impacted by lower temperatures, losing up to 50% of their Ah capacity. This is in addition to the fact that only about 80% of the batteries stated Ah capacity can be utilised before their volatage output drops below the stated level.

In contrast, the more expensive LiFeP04 batteries can utilise almost 100% of their stated Ah capacity whilst maintaining their voltage, and lower temperatures have little to no impact on their performance.

If we then move to the voltage conversion, converting a batteries DC power to an AC output requires a power invertor. Most power invertors have an efficiency of around 80%, so, for every 1Ah of capacity our battery has, only 0.8Ah remains once they voltage has been inverted. Therefore our battery size requirement needs to take this efficiency loss in to account when calculating the number of days power our solar system can deliver without solar charge.

Once we know the device(s) power consumption in Ah and the required battery size, based on the number of days power we want it to provide without solar charge, we can then move to the solar panel size and charge controller.

A battery can be safely charged at an amp rating of 10% of the batteries Ah capacity. Therefore a 100Ah can be charged at a 10amp rate.

To select a suitable solar charge controller, we must ensure that the controller has the correct charging capability for the battery technology chosen. Most solar charge controllers can charge sealed lead acid and AGM batteries, either automatically detecting the battery technology or by programmable battery type selection. For LiFeP04 batteries you must ensure you choose a solar charge controller that either has pre-programmed or programmable LiFeP04 charging parameters. Charging an LiFeP04 battery using sealed lead acid or AGM battery parameters will damage the battery and significantly reduce its performance and lifetime.

Unfortunately solar charge controllers are not 100% efficient. They consume a small amount of power themselves and their efficiency ranges from 75% to 98% dependent on the charge controller type and quality.

MPPT solar charge controllers have an efficiency of circa 93-95%, however some do have a stated efficiency of up to 98%.

PWM solar charge controllers have an efficiency of circa 75%.

So what does this mean for our solar panel size?

Solar panels are rated in Watts. We therefore need to convert our battery size from Ah to Wh. As we know the Ah consumption of our device(s) we can easily calculate the % of the battery capacity that they will consume per day. We then convert this to Wh using the calculation below.

Ah x Voltage = Wh

If our device requires AC power, we must also take in to account the efficiency loss of the power invertor.

An example of the Wh conversion is shown below.

Device power consumption = 3w @ 12v
Ah required per day = 6Ah
Battery size and technology = 100Ah LiFeP04
Battery voltage = 12v
Daily battery % consumed = 6%
Battery charge required in Wh = 72Wh

Theoretically an 80W solar panel will deliver 80W of charge for every 1hr of peak sunlight the solar panel is exposed to. The number of peak sunlight hours varies significantly throughout the year and by geographical location. For the purpose of calculating a solar power system's suitability we can use the average peak sunlight hours per day as published by the Met office. However, these peak sunlight hours cannot be guaranteed and will vary from day to day / year to year. Therefore we must build in a tolerance to ensure our system will continue to perform in the worst case scenario.

We would always recommend monocrystalline solar panel technology is chosen.

Based on the example above and adding a 70W solar panel, we can calculate that theoretically our system will need 1.03 hrs of peak sunlight to fully recharge the battery each day.

Unfortunately solar panel charging has its own efficiency loss. This is stated by The National Renewable Energy Laboratory as being 14.08%. We must therefore build in this efficiency loss in to our calculations.

This increases our systems peak sunlight requirement from 1.03hrs to 1.2hrs per day.

So how do we know if our system will receive enough solar charge throughout the year to keep our system online?

The way we can do this is to run our solar power system and solar charge requirement through a 365 day model using the Met Office published average peak sunlight hours per day.

We also need to build into this model any reduction in the systems optimal installation.

The optimal installation for any solar systems is

Solar panel direction = Due South

Solar panel angle = 45°

Skyline restriction = None

Any variance from the optimal solar panel installation will reduce the system's charging performance.

We have developed a handy solar power tool to run the 365 modelling. By entering the solar power configuration and device power consumption, the tool will run through the device's power consumption and solar charge each day using the Met Office average peak sunlight hours as the assumed solar charge achieved. Once the modelling is complete, the result will show if the battery has at any point reached 0% charge (or 20% charge in the case of sealed lead acid and AGM batteries). The tool will return the date on which the battery will be depleted and detail how many days the solar charge will be insufficient to provide battery charge to power the device for 24 hours. The tool will run the model forward from "today" (today being the date the tool is used).

If your solar installation will not be positioned with the optimal installation, the site below can be used to calculate the achieved average sunlight hours per day and can be manually entered into our modelling tool.

Equally if your solar panel will be partically obscured by buildings, foliage, infrastructure or any other element impacting the panels clear skyline, the percentage of restricted skyline can be entered to reduce the available average sunlight hours per day. Please bare in mind that these calculations are based on the % reduction entered and the tool does not take in to account the position of the restriction and therefore the impact this may have on the solar panels exposure to peak sunlight time. For example a restriction due South of the solar panel will have a greater reduction on available sunlight hours than a restriction to due East or West.

AndrewMarsh.com has a fantastic sun path visualisation tool which adjusts the path based on geographical location.

Sun Path 3D Visualisation Tool

Some key considerations when selecting a solar power system size are:

The power consumption of device(s) must be accurate, as any inaccuracy in this information may significantly impact the calculations and suitability of the solar power system chosen.
The average peak sunlight hours cannot be guaranteed. Although helpful, the information supplied is based on historic average sunlight hours and is provided for guidance purposes only.
Efficiency loss can be a major factor in the size of solar power system required. Opting for less expensive solar charge controllers, solar panel or battery technology can significantly reduce a systems efficiency and increase the number of peak sunlight hours required to recharge the battery.

To purchase our solar power systems please get in touch or speak with one of our distribution partners.